The intermittent control of pressurized medium in slip-controlled brake systems by digitally switchable inlet and outlet valves leads to undesired sound emission as a consequence of the pulsed changes in pressure.
It was found in the brake systems described in German Patent Application No. P 43 19 227.0 that the solutions suggested for arranging the annular piston provided with a switchable diaphragm in the inlet valve do not always make it possible to avoid an unintended switching of the annular piston into the diaphragm position during manual, slip-free brake actuation. The annular piston having a "switchable diaphragm" moves between a first position and a second position (i.e., "diaphragm position"). At the first position, a pressurized medium flow path across a face of the annular piston extends unhindered from the valve seat to a pressure line, in parallel to the diaphragm. At the second position, this pressurized medium flow path is closed and pressurized medium flows from the valve seat to the pressure line exclusively through the diaphragm. An undesired premature activation of the switchable diaphragm associated with the inlet valve cannot be ruled out with sufficient certainty, especially in the case of rapid actuation of the brake (e.g., panic braking). In the event of such premature activation of the switchable diaphragm, the feel of the pedal and the vehicle-specific pressure build-up gradient will change. A reduction in the pressure build-up gradient brought about by the diaphragm action inherently leads to a reduction in braking power.
It was therefore proposed in German Patent Application No. P 43 32 819.9 that fixed diaphragms be additionally arranged upstream or downstream of the solenoid valve (inlet valve), but the result is that the switchover pressure or the switching pressure difference of the switchable diaphragm remains relatively high, which may lead to undesired fluctuations in the volume flow during the brake pressure control.